Before barbed wire, stockmen couldn't fence enough acreage to support their livestock. Instead, a system of open range grazing evolved. Cattle were turned out on public land and left to graze wherever they found grass. Limited in their roaming only by rivers, rough country, or waterless stretches, the cattle might spread over a million acres. Cattle from many owners mingled, leading to the establishment of round associations and grazing districts. As the open range system expanded north from its roots in the Southwest, American cowboys learned herding, roping, and other skills from the Spanish vaquero, even adapting that word to buckaroo.